Saturn's third largest moon Dione can be seen through the haze of its largest moon, Titan, in this view of the two posing before the planet and its rings from NASA's Cassini spacecraft. Credit: NASA/JPL-Caltech/SSI

Saturn's giant moon Titan hides behind a thick, smoggy atmosphere that's well-known to scientists as one of the most complex chemical environments in the solar system. It's a productive "factory" cranking out hydrocarbons that rain down on Titan's icy surface and cloak it in soot. With a brutally cold surface temperature of around –270° Fahrenheit (–170° Celsius), the hydrocarbons form lakes of liquid methane and ethane.

However, the most important raw ingredient in Titan's chemical factory is methane gas. Methane is a molecule made up of one carbon atom joined to four hydrogen atoms. It should not last long because it's being continuously destroyed by sunlight and converted to more complex molecules and particles. New research from NASA-funded scientists attempts to estimate how long this factory has been operating.

This new research used data collected by two instruments aboard NASA's Cassini spacecraft in orbit around Saturn and one instrument on the European Space Agency's Huygens probe that landed on Titan's surface in January 2005. A study led by Conor Nixon from the University of Maryland, College Park, uses infrared signatures — spectra — of methane from Cassini's composite infrared spectrometer to estimate how much "heavy" methane containing rare isotopes is present in Titan's atmosphere.

Isotopes are versions of an element with different weights, or masses. For example, carbon-13 is a heavier — and rare — version of the most common type of carbon called carbon-12. Occasionally, a carbon-13 atom replaces a carbon-12 atom in a methane molecule. Because methane made with carbon-12 is slightly lighter, the chemical reactions that convert it to more complex hydrocarbons happen a bit faster. This means carbon-12 methane gets used up at a slightly faster rate than heavy carbon-13 methane, so the concentration of heavy methane in Titan's atmosphere increases slowly.

By modeling how the concentration of heavy methane changes over time, the scientists predicted how long Titan's chemical factory has been running.

"Under our baseline model assumptions, the methane age is capped at 1.6 billion years, or about a third the age of Titan itself," said C. A. Nixon from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. "However, if methane is also allowed to escape from the top of the atmosphere, as some previous work has suggested, the age must be much shorter — perhaps only 10 million years — to be compatible with observations."

Both scenarios assume that methane entered the atmosphere in one burst of outgassing, probably from the restructuring of Titan's interior as heavier materials sank toward the center and lighter ones rose toward the surface.

"However, if the methane has been continuously replenished from a source, then its isotopes would always appear 'fresh,' and we can't restrict the age in our model," said Nixon. Possible sources include methane clathrates, basically a methane molecule inside a "cage" or lattice of ice molecules. Methane clathrates are found in the frigid depths of Earth's oceans, and some scientists think there could be an ocean of liquid water mixed with ammonia (acting as antifreeze) beneath Titan's water-ice crust. If this is so, methane might be released from its clathrate cages during the eruptions of proposed “cryovolcanoes” of water-ammonia slurry, or more simply could slowly seep out through fractures in the crust.

The second Titan study by Kathleen Mandt from the Southwest Research Institute in San Antonio, Texas, and colleagues also models the time-evolution of methane. In this work, the concentration of the heavy methane is determined from measurements by Cassini's ion and neutral mass spectrometer, which counts molecules in the atmosphere of different masses. Measurements made by the Huygens gas chromatograph mass spectrometer, which also counts molecules of different masses, were used to constrain the impact of escape on the heavy methane in the atmosphere.

"We compute that even if methane has been replenished from the interior over time to match or exceed the amounts fed into the atmospheric chemical factory, the process must have been running for a maximum of 1 billion years," said Mandt. "If the process had started any earlier, we would see a buildup of methane in the lakes on the surface and in the atmosphere beyond what is observed today."

Together, these papers add important new perspectives and constraints on the history of Titan's methane atmosphere, confirming that it must have formed long after Titan itself. Previous work considering the evolution of Titan's interior predicted the last major methane eruption occurred 350 million to 1.35 billion years ago, while crater counting has put the age of the current surface at 200 million to 1 billion years. (Crater counting works on the principle that an older surface has more craters.)

The present work for the first time estimates the methane age from the atmosphere itself, at less than 1 billion years.